A) Physical factors: The soil and atmosphere are the chief physical factors which determine the flow rate of water through plant.

Soil factors:

i) Soil water content: The plant roots can easily absorb the soil moisture in between field capacity and permanent wilting point. When the soil moisture decrease below the wilting point, plant roots have to exert more pressure and thus rate of absorption decreases. On the other hand, when the soil is completely saturated with water, then soil temperature and aeration are poor and this condition also affects the absorption of water.
ii)Soil temperature: Soil temperature is known to influence water absorption and ultimately transpiration to a considerable extent. In many plants, water absorption below a soil temperature of 10 oC is reduced sharply and 25 oC soil temperature up take of water is slowed down. In most instances, temperature above 40 oC does not support water absorption and plant can show signs of wilting. A freezing temperature reduces water absorption because of following causes.
a) Decreased root growth
b) Increased viscosity of water
c) Increased resistance to movement of water in to roots. thus is caused by decreased permeability of cell membrane and the increased viscosity.

iii) Soil aeration and flooding: Most of crop plants are not able to water while standing under water logged conditions. The following are the possible reasons of flood injury.
a) Poor availability of oxygen and occurrence if higher CO2 concentration around roots.
b) Accumulation of toxic substances either in the submerged roots or around them.
c) Changes in pattern of ion up take resulting in the accumulation of some toxic ions.

In water logged condition, the availability of oxygen is reduced which affects respiratory actively of roots. In addition, CO2 concentration is increased and it affects permeability of membranes and adversely influences water up take. Reduced oxygen also affects root growth adversely.

B) Atmospheric factor:

Classification of Crops According To Root Depth, Rooting Characteristic And Moisture Use Of Crops.

The amount of soil moisture that is available to a plant is determined by the moisture characteristics of the soil, the depth to which the plant roots extend and the proliferation or density of the roots. Soil moisture characteristics, such as field capacity and wilting percentage are peculiar to a soil and are a function of the texture and organic matter. Little can be done to alter these limits to any great extent. Greater possibilities lie in changing the characteristic of the plant enabling it extend its rooting system deeper into the soil, thereby enlarging its reservoir of water. The density of roots proliferation is important.

Water is an unsaturated soil moves very slowly, and only a distance of a few cm. To utilize effectively the moisture stored in the soil profile, roots must continue to proliferate into unexploited zones throughout the plants growth cycle. During favorable growing periods, roots often elongate so rapidly that satisfactory moisture contacts can be maintained even when the soil moisture content declines. Where transpiration is effected due to the different atmosphere factors such as wind velocity, humidity, sunlight, etc when temperature and wind velocity are more sunlight for longer period and humidity are less, under such conditions, transpiration is more. The increased rate of transpiration results more water uptake.

C) Biological factors:

Root system is the plant factor which is directly related to the absorption of water from soil. Under favorable soil water, potential soil temperature, aeration, and roots system of the plants strongly influence the uptake of water. When growth of roots (root system) is more, uptake of water is also more under favorable soil conditions. Root growth is influenced by soil and more therefore agronomic management practices can help to improve root growth.

Other plant factors such as morphology of leaves, stomatal mechanism and growth stage of the crop influence the rate of transpiration. The increased rate of transpiration results more water absorption.

Good root system has developed during favorable growing periods; a plant can draw its moisture supply from deeper soil layers.

Plants vary genetically in their rooting characteristics. Vegetable crops such as onions and potatoes have a spare rooting system and are unable to use all the soil water within the root zone. Forage grasses, sorghum, maize and such other crops have very fibrous, dense roots. Lucerne has a deep root system. Whether plant is an annual or perennial is another factor affecting its its moisture relations. An annual plant must extend its roots down into the soil to make availability root depth, and needs only to extend its small roots and hairs to be able to utilize the entire amount of available soil water.

Plants may be limited in their rooting by factors other than genetic. High water table, shallow soils and an impermeable formation near the ground surface restrict the depth rooting. Fertility and salt status of the soil influence the rooting of plants crop management practices, such as cutting the top growth at different physiological stages and the cultivation and cutting of surface roots after rooting habits. The rooting pattern of common and crop plants vary widely from soil. For example, roots of maize crop have been found to extend as deep as 1.5 meters in medium to textured soils, while in a fine textured soil the crop has a shallower root system.

Effective Root zone: Effective root zone is the depth from which the roots of average mature plant are capable of reducing soil moisture to the extent that it should be replaced by irrigation. It is not necessarily to have maximum root depth for ant given plant especially for plants that have a long taproot. Root development of any crop varies widely with the type of soil and other factors.

Table: Effective root zone depth of some crops and their classification.

Rooting Characteristic

Shallow Rooted

Moderately Deep Rooted

Deep Rooted

Very Deep Rooted

Rice

Wheat

Maize

Sugarcane

Potato

Castor

\Cotton

Citrus

Cauliflower

Ground Nut

Sorghum

Coffee

Cabbage

Pea

Bajara

Apple

Lettuce

Bean

Soybean

Grape Vine

onion

Chili

Sugar Beet

Safflower

Tobacco

Tomato

Lucerne

Moisture extraction pattern within root zone

The moisture extraction pattern shows the relative amounts of moisture extracted from different depths within the crop root zone.

It is seen that about 40 percent of the total moisture used is extracted from first quarter of the root zone, 30 percent from the second, 20 percent from third and only 10 percent from last quarter.

This indicates that the need for making soil moisture measurements at different depths within the root zone in order to have estimate of soil moisture status.